This invention relates to a knock deriving apparatus for an internal combustion engine, and in particular to an apparatus for generating knock signals which are used in the control of the ignition timing of the engine.
As a method for detecting knocks arising in an internal combustion engine, it is generally known in the art to use such a method as, to detect mechanical engine vibrations, to detect the inner pressure of the combustion cylinders, or to detect sounds generated in the engine. Since these methods detect high frequency vibrations due to the occurrence of knocks by means of a detector mounted on or near the engine, the detector detects not only a knock signal component but also a noise signal component such as the mechanical vibrations of the engine and/or the variations in the combustion pressure of the engine. Therefore, in order to detect the occurrence of knocks, it is necessary to discriminate a knock signal from the output signal of the detector comprising the knock signal component and the noise signal component.
For the discrimination of the knock signal, a method using the characteristics of the knock signal is effective. For example, a knock signal is known to have a specific frequency component so that a method of discriminating, in frequency, the knock signal from a noise signal has been conventionally used.
FIG. 1 shows a frequency spectrum of an engine vibration signal detected in an engine, in which "A" indicates a spectrum portion where no knock occurs and "B" indicates a spectrum portion where knocks occur. It is seen from FIG. 1 that the portion "B" has a signal level higher than the portion "A" over a frequency band width fb about a frequency fo as a center. Therefore, by passing the above noted engine vibration signal through a band-pass filter having the band width fb with the central frequency fo, the noise signal component in the frequency band other than that of the knock signal component can be removed.
FIG. 2 shows a signal waveform as a function of time which has just passed the above noted band-pass filter, in which a portion "C" denotes a knock signal component having a high amplitude, and a portion "D" denotes a noise signal component having a damped, i.e. low amplitude because of the band-pass filter filtering out only a signal component within the above noted pass band. Accordingly, by the comparison of a comparative reference signal, which has a signal level which is higher than that of the noise signal component and is lower than that of the knock signal component, with the output signal of the band-pass filter, the knock signal component can be fully discriminated from the noise signal component, so that the knocks can be derived.
FIG. 3 shows a block arrangement of a prior art knock deriving apparatus, having such a discriminating function as noted above, which is described in the U.S. Pat. No. 3,822,583. In the figure, a knock sensor 10 for detecting knocks of an internal combustion engine is connected to a band-pass filter 12 which passes therethrough a frequency component specific to knocks in the output signal of the knock sensor 10. The output of the band-pass filter 12 is connected to the input of a comparative reference generator 14 which develops a comparative reference on the basis of the output signal of the band-pass filter 12. The outputs of the band-pass filter 12 and the comparative reference generator 14 are both connected to the inputs of a comparator 16 which compares the two signals.
In FIGS. 4A-4C are shown dynamic waveforms of the knock deriving apparatus shown in FIG. 3. The waveform shown in FIG. 4A shows the output signal of the knock sensor 10 in the case where an engine vibration sensor is employed as the knock sensor 10. As can be seen from this waveform, the knock signal is little different from the noise signal in amplitude so that no discrimination is possible based on the amplitudes thereof. A waveform I shown in FIG. 4B shows the output signal of the band-pass filter 12 which has a damped noise signal component as above described and, accordingly, has a good S/N ratio. A waveform II shown in FIG. 4B shows a comparative reference signal outputted by the comparative reference generator 14. It is to be noted that the comparative reference generator 14 may be formed of, for example, a rectifier circuit 141, a CR averaging circuit 142, and an amplifier circuit 143 as shown in FIG. 8, in which the output signal of the band-pass filter 12 shown by the waveform I in FIG. 4B is rectified and averaged to provide a DC signal which is then amplified at a predetermined amplification to provide the comparative reference.
The comparator 16 compares the output signal (waveform I) of the band-pass filter 12 with the output signal (waveform II) of the comparative reference generator 14. Since the above-mentioned amplification is set so that only the knock signal component may exceed the comparative reference, the comparator 14 outputs a pulse train as shown in FIG. 4C. Therefore, depending upon the presence or absence of the pulse train, as shown in FIG. 4C, outputted from the comparator 16, the occurrence of knocks can be detected.
However, should any of the mechanical parts in the engine produce a resonant noise which is at the same frequency as the knock frequency and has a large amplitude variation, since such a noise is detected by the knock sensor 10, a noise signal component having a large amplitude variation is outputted from the band-pass filter 12. Accordingly, the comparative reference signal outputted from the generator 14 varies to a large extent as the noise signal component varies. This state is shown by a portion "E" in FIG. 4B.
On the other hand, the U.S. Pat. No. 4,111,035, issued to G. A. West and G. C. Hamren on Sept. 5, 1978, entitled "Engine Knock Signal Generating Apparatus with Noise Channel Inhibiting Feedback" discloses a way to obtain such a comparative reference voltage on the basis of the output signal of a knock sensor i.e., not on the basis of the output signal of a band-pass filter.
However, such prior art apparatuses are disadvantageous in that if there is a noise source such as noted above, the comparative reference signal varies too much and it becomes difficult to precisely derive or generate the knock signal component.